Cooling systems are generally activated when the temperature of a room or zone being cooled by the system is above a desired set point temperature, and deactivated when below. Under low load conditions, such as at night when the outdoor temperature drops, the frequency and length of time the system is activated greatly decreases. If the system is left deactivated for an extended period, a lack of both dehumidification and air circulation may make the room air feel uncomfortable, even though the room temperature is at or near the set point.
For greater comfort, some systems include a humidistat and are responsive to both cooling and dehumidification demands. Humidistats, however, have several drawbacks. They are not always reliable, they increase the cost of the control, and they are often unresponsive to ventilation needs.
Other systems place a small resistive heating element near the thermostat's temperature sensor. The heating element is energized when the cooling system is off. The thermostat reacts to the heat by energizing the cooling system more frequently than if the heating element were not used. The heating element, however, also adds unnecessary cost to the control and often involves trial and error manual adjustments.